1. Field of the Invention
The invention relates to a brake booster, and more particularly, to a brake booster in which a key member which is mechanically coupled to a valve plunger is brought into abutment against an internal wall surface of a shell, for example, when the booster is inoperative, to thereby limit a free retracting movement of the valve plunger and an input shaft which is mechanically coupled therewith in order to reduce a lost stroke of the input shaft when the brake is applied for the next time.
2. Description of the Prior Art
A brake booster is known in the prior art which comprises a valve body slidably disposed within a shell, a valve mechanism received within the valve body, an input shaft mechanically coupled to a valve plunger which forms part of the valve mechanism to switch a flow path within the valve mechanism, and a key member which is mechanically coupled to the valve plunger, with the key member being brought into abutment against an internal wall surface of the shell to limit a free retracting movement of the valve plunger when the booster is inoperative, thereby reducing a lost stroke of the input shaft when the brake is applied for the next time.
It will be appreciated that a shell of a brake booster is generally formed with a cylindrical extension which is formed by a rear central portion of the shell to extend in a cylindrical form and to remain open. A rear cylindrical portion of a valve body slidably extends through the cylindrical extension of the shell to the outside thereof. A seal member is disposed within the cylindrical extension to maintain a hermetic seal between the extension and the cylindrical portion of the valve body. The key member mentioned above is disposed for abutment against an internal wall surface of the shell which is located radially outward of the cylindrical extension.
It is also known in the prior art to dispose the key member for abutment against a bearing which is disposed within the cylindrical extension to guide the rear cylindrical portion of the valve body or against a stop member which is disposed within the cylindrical extension, rather than against the internal wall surface of the shell at a location which is radially outward of the cylindrical extension (see Japanese Laid-Open Patent Application No. 198,158/1982).
However, when the key member is disposed for abutment against an internal wall surface of the shell at a location which is radially outward of the cylindrical extension, there must be a space for the provision of the seal member between the surface against which the key member abuts and the outer peripheral surface of the valve body, causing an increased length of the key member by a corresponding amount. This requires that the wall thickness be increased in order to secure a required strength, resulting in an increase in the weight and the axial length of the brake booster.
On the other hand, in an arrangement in which the key member is disposed for abutment against the bearing located within the cylindrical extension, the length of the key member can be reduced with a resulting reduction in the wall thickness and the weight. However, the abutment of the key member against the bearing may cause a degraded durability of the bearing. The provision of a stop member within the cylindrical extension is effective to prevent any reduced durability of the bearing, but results in an increased cost because of an added member.